1. Field of the Invention
The present invention relates to a key unit used for a mobile device such as a movable telephone and a personal digital assistance terminal (PDA), and more particularly to a key unit comprising a hard base made of a hard resin plate.
2. Description of the Related Art
A key unit is one of the components constituting a mobile device such as a movable telephone and a PDA, and includes a number of switch operation keys (pushbutton) collectively arranged on one sheet surface. Each key includes a key top made of hard resin, which is firmly fixed to a surface of a key pad made of flexible rubber, and a switch thrusting projection (so-called “thruster”) provided on its backside. The keys are connected to each other through the key pad. By adhering a printed-circuit board having switch elements to an undersurface of the key unit thus constructed, a key switch is formed at a position corresponding to each key. The conventional art has employed a structure, where the key top on the key unit is viewed from a key window provided on a casing surface of a movable telephone or the like.
As a result of a recent progress in miniaturization of a movable telephone or the like, there has been a stronger demand for a thinner key unit, and a distance from a key top upper surface to the key unit undersurface has been excessively shortened. In such a thin key unit, an end of the key top gets under a key window frame of the casing when the key top is pushed down, consequently creating a problem of impossible returning of the pushed down key.
The foregoing problem may be attributed to the facts that the entire key unit is made of soft and elastic rubber, shape stability is lacking because of the excessively thin key unit, and the key top is slid in a cross direction because of deformation of the key unit during key pushing down. In addition, such a soft key unit has a difficulty of being subjected to automatic assembling.
As measures to deal with the getting-under of the key top, a key unit similar to that shown in FIG. 8(A) has heretofore been presented, which includes a ring-shaped projection (rib) provided around a center on the backside of the key top. In the case of an illuminated key, however, another problem occurs, that is, a shadow of the ring-shaped projection is seen from the key top upper surface, consequently spoiling the beauty.
In the conventional key unit, a structure similar to that shown in FIG. 8(B) has been employed, in which an light guiding plate for key top illumination is superposed on the key unit. However, this dual structure is contradictory to the demand for a thinner key unit.
On a keyboard of the rapidly developed PDA or the like, a need to arrange a number of keys on a narrow area has created a tendency to employ a so-called narrow pitch key unit, in which a space between the keys is extremely narrow. On such a keyboard, since there is no space for providing a key window frame similar to the above between adjacent key tops, positioning of the key tops must be carried out only by the soft key pad. In the key unit of such a structure, problems easily occur. For example, the key top catches on something to float, and pushing down of one key is transmitted to an adjacent key, causing interferences with movements of each other.